What to do with the leftovers?

I'm always up for trying out new things with leftovers, as I hate to waste food, but my greed always forces me to make too much.  Last night I still had quite a large amount of a Beef joint leftover from the meal the night before and really did not fancy just making a roast with it, as of course I wanted to do that on Sunday  like I said it was very large joint).  I only really had pasta or potatoes to work with too.  But the resulting dish was a success and this is always a moment of joy for me when that happens.  It may need a little tweaking as I used ingredents that just happened to be around but a pleasure to eat all the same.  I will take a picture next time that this dish comes my way as I was a little rushed to feed everyone last night, but so excited I just had to share it.  Quanities are not very precise as I was winging it a bit and didn't weight anything, so just adjust to how much you need and to taste.

In a large pan bring water to the boil and add the Pasta of your choice.  Once in, melt some Butter in a seperate pan and then add Flour to make a roux, about 2 tbsp of each.  Then slowly add in Beef Stock until to get a sauce that is just a bit thicker that double cream.  In another pan heat 1 tbsp of Butter and Oil and Gently soften Half an Onion, sliced.  Once softened add 1/2 tsp Mustard Powder, the leftover cooked Beef, cut in to strips, 1 Clove of Garlic, 2 tsp Fresh Tyhme, a good couple of handfuls of sliced Mushrooms and plenty of Black Pepper and heat through.  Just before the pasta is ready add some Cream to the sauce, I used a good dollop of Clotted Cream, as that's all I had in the fridge, but Double Cream would be great also.  Drain the pasta and return to the large pan, add in the Beef and Sauce and mix well.  Serve x